1. Field of the Invention
The present invention relates to an apparatus for disposing of fuel vapor, and more particularly, to an apparatus for disposing of fuel vapor which improves drivability of a vehicle by shortening an ON-OFF interval of a duty control valve under a predetermined operating condition of an engine, or by equalizing engine output torque in such a manner that an appropriate amount of fuel is injected into each cylinder of the engine or that the ignition timing of each cylinder is adjusted, wherein the appropriate amount of fuel or the ignition timing is determined by a method having the steps of calculating traveling time of a vapor laden air (purged gas) from the duty control valve to an intake port of the engine and detecting the cylinder into which the purged gas is sucked.
2. Description of the Related Art
Generally, internal combustion engines are provided with an evaporative emission-control system which prevents escape of fuel vapor evaporated from a fuel tank and a carburetor into the open air when the engine is stopped. The evaporative emission-control system has a canister filled with an adsorbent such as an activated charcoal, for trapping fuel vapor (HC) from the fuel tank and the carburetor. In the evaporative system, the fuel vapor trapped in the canister is sucked into the combustion chambers of the engine due to the negative air pressure generated by the engine during is running.
As one such evaporative system, there is an apparatus for disposing of fuel vapor which is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-174557. This apparatus is provided for the purpose of properly mixing air-intake with fuel vapor, suppressing fluctuations in air-fuel ratio, and improving the stability of output torque of an engine. The apparatus executes a process, namely, a purging process which returns fuel vapor trapped in the canister back to the air-intake side of the engine. The apparatus determines a duty ratio of an electromagnetic duty control valve, to change the flow rate of the purged gas in accordance with engine operating conditions and determine a driving frequency of the duty control valve which is inversely proportional to the duty ratio. The apparatus actuates the electromagnetic valve in accordance with the determined duty ratio and the driving frequency, thus controlling the purging process. However, although the air-intake and fuel vapor can be properly mixed, there is a problem of noise occurrence due to the activation of the electromagnetic valve since the duty ratio is set low and the driving frequency of the electromagnetic valve is set high when engine load is light such as idling time.
To solve this problem, there is provided another apparatus for disposing of fuel vapor which is disclosed in Japanese Unexamined Utility Model publication (Kokai) No. 4-1658 and entitled "Canister Purge Controller". This apparatus for disposing of fuel vapor sets the duty ratio of a purge control valve which purges a minimum required volume of the purged gas at idling time in order to reduce noise occurrence at idling time, and it also reduce the noise at idling time by setting the driving frequency of the electromagnetic duty control valve low.
However, the above mentioned apparatus for disposing of fuel vapor sets a low driving frequency of the electromagnetic duty control valve (DVSV) only at engine idling time. When the duty control valve (DVSV) periodically opens and closes even at engine idling time, the air-fuel ratio of the engine periodically changes such that the air-fuel ratio becomes rich when the duty control valve (DVSV) is opened, and the ratio becomes lean when the duty control valve (DVSV) is closed. This fluctuation of the air-fuel ratio increases proportionally to the concentration of the purged gas, destabilizes the engine combustion state and changes the engine output torque, thereby deteriorating the drivability of the vehicle and creating exhaust emissions. Furthermore, the higher driving frequency of the DVSV, reduces the lifetime of the DVSV valve.
Regarding the concentration of the purged gas, there is a problem that the engine combustion state changes due to the fluctuation of the air-fuel ratio caused by an increase of the concentration of the purged gas. Regarding an apparatus for disposing of fuel vapor by using an EGR (Exhaust-gas recirculating) system, there is a problem in that the engine combustion state changes depending on the volume of exhaust gas recirculating in the EGR system. In an automatic transmission (A/T) vehicle equipped with a torque converter, the converter absorbs torque fluctuation of the engine when the torque converter is not locked, however drivability of the vehicle is deteriorated because of resonance which occurs when the torque converter is used and the period of torque fluctuation of the engine is close to a resonant frequency in the driving mechanism of the vehicle.
Furthermore, an apparatus for disposing of fuel vapor according to the prior art destabilizes the engine combustion state and fluctuates the output torque of the engine when the volume of the purged gas sucked into each cylinder of a multi-cylinder engine is very different between each of the cylinders, thereby deteriorating the drivability of the vehicle and creating exhaust emissions in the same way as the previously described prior art apparatus.